The present invention relates to a bearing support structure for installation in a gas turbine engine between radially inner first and second bearings adapted to rotatably support a first and a second rotor, respectively, and a radially outer engine frame, the support structure comprising a first annular plate-shaped part and means for supporting the first bearing at one end thereof and a second annular plate-shaped part and means for supporting the second bearing at one end thereof. The invention also relates to a gas turbine engine comprising the bearing support structure.
The bearing support structure may be used in stationary gas turbine engines, but is especially advantageous for aircraft jet engines. Jet engine is meant to include various types of engines, which admit air at relatively low velocity, heat it by combustion and shoot it out at a much higher velocity. Accommodated within the term jet engine are, for example, turbojet engines and turbo-fan engines. The invention will below be described for a turbo-fan engine, but may of course also be used for other engine types.
An aircraft gas turbine engine of the turbofan type generally comprises a forward fan and booster compressor, a middle core engine, and an aft low pressure power turbine. The core engine comprises a high pressure compressor, a combustor and a high pressure turbine in a serial relationship. The high pressure compressor and high pressure turbine of the core engine are interconnected by a high pressure shaft. The high-pressure compressor, turbine and shaft essentially form a high pressure rotor. The high-pressure compressor is rotatably driven to compress air entering the core engine to a relatively high pressure. This high pressure air is then mixed with fuel in the combustor and ignited to form a high energy gas stream. The gas stream flows aft and passes through the high-pressure turbine, rotatably driving it and the high pressure shaft which, in turn, rotatably drives the high pressure compressor.
The gas stream leaving the high pressure turbine is expanded through a second or low pressure turbine. The low pressure turbine rotatably drives the fan and booster compressor via a low pressure shaft, all of which form the low pressure rotor. The low pressure shaft extends through the high pressure rotor. Most of the thrust produced is generated by the fan. Engine frames are used to support and carry the bearings, which in turn, rotatably support the rotors. Conventional turbo fan engines have a fan frame, a mid-frame and an aft turbine frame.
The engine is mounted to the aircraft at a forwardly located fan frame forward mount on the fan frame and at a rearwardly located turbine frame aft mount on the turbine frame.
The invention especially relates to the aft portion of the engine, and more particularly where an aft portion of the engine frame is located between the high pressure turbine and the low pressure turbine and extends radially inwardly from the position of a turbine frame aft mount. There are known solutions of bearing support structures arranged radially between and connected to inner first and second bearings which rotatably support a first and a second rotor, respectively, and the radially outer aft portion of the engine frame. According to one such solution, the bearing support structure comprises a casted bearing house supporting both bearings and which is fixedly joined to a so-called torsion box, either by welding or bolts. The torsion box in turn is fixedly joined to the engine frame. Such a casted bearing house often has a complex shape and is casted in one-piece. The process for producing such a bearing house and to mount it in the engine is time-consuming.
A further example of a prior art aircraft engine is described in U.S. Pat. No. 6,708,482. An aft portion of the engine frame is located between the high pressure turbine and the low pressure turbine and extends radially inwardly from the position of a turbine frame aft mount. A bearing support structure supports the first and second bearing which rotatably support the first and second rotor, respectively. The bearing support structure comprises two separate plate-shaped parts, each extending from one of the first and second bearing to the engine frame. The two separate plate-shaped parts are bolted to the engine frame from opposite sides of it in the direction of the engine longitudinal central axis. The position of the bearings and the construction of the bearing support structure makes installation of the structure in the engine and removal of it from the engine cumbersome. Further, there is a trend to move the rearwardly located turbine frame aft mount forward so that the complete, or at least parts of the, low pressure turbine receives an overhang. One reason for this development is to cut weight. The traditional aircraft engine Turbine Rear Frame (TRF), also called Tail Bearing House (TBH) or Turbine Exhaust Case (TEC), will then be replaced by or complemented with a bearing support structure forwardly of the complete, or at least parts of the, low pressure turbine. This bearing support structure, so-called Turbine Center Frame (TCF) or Turbine Mid-Frame (TMF), will transfer bearing loads from at least two different shafts. Large stiffness requirements are placed on the bearing support structure and the associated aft engine frame due to dynamic interaction between these shafts, which may rotate in the same direction or be counter-rotating.
It is desirable to achieve a bearing support structure for a gas turbine engine with two rotors, which structure should be easily installed and have a sufficient stiffness for transferring bearing loads and be positioned radially between bearings for the two rotors and an engine frame. The bearing support structure should preferably be of light weight. The bearing support structure should further be especially suitable for a gas turbine engine with its rearwardly located turbine frame aft mount positioned forward so that the complete, or at least parts of the, low pressure turbine receives an overhang.
By using two plate-shaped parts for supporting the two bearings, the complex prior art casted bearing house is no longer required. Further, this solution creates conditions for eliminating the prior art torsion box. Thus, the one-piece bearing support structure replaces the casted bearing house and the torsion box. Further, the invention creates conditions for using stronger materials in the bearing support structure and it can also be made lighter relative to conventional castings.
One requirement on gas turbine engines is that they should withstand large rotational inbalances which may occur when a fan or turbine blade, or parts thereof, comes loose for some reason. The inventive solution has great potential in that the effects of an inbalance on one of the bearings may be transferred to the other bearing through the two plate-shaped parts. This results in smaller reactional forces in engine mounts, which creates conditions for making a lighter engine frame.
Each of the bearing support means of the plate-shaped parts comprises a bearing holder or race integrated in or joined to the respective plate-shaped part. The bearings are mounted in the respective bearing holder or race in the support portions.
According to a preferred embodiment of the invention the bearing support structure comprises an annular central member and that the first and second plate-shaped parts are fixedly joined to the annular central member. This creates conditions for a sufficiently stiff structure for transferring loads between the bearings and the engine frame. The first and second plate-shaped parts are preferably conical and diverge from the central annular member to the respective bearing.
According to a further development of the last-mentioned embodiment, the structure comprises a third annular plate-shaped part with means for connection to the engine frame. Also the third plate-shaped part is preferably fixedly joined to the annular central member.
Such a construction further improves the stiffness of the structure.
The bearing support structure preferably also comprises a fourth annular plate-shaped part with means for connection to the engine frame. By joining a first end of each of the four conical plate-shaped parts to the central member and arranging the parts to extend in different directions forming the shape of an X in cross section transverse to the engine longitudinal central axis, radial stiffness between the bearings and the engine frame is achieved. Further, a flexibility for relative motion between the two bearings is achieved. Further, a torsional moment on the engine frame is minimized, which creates conditions for making this component lighter and less complex.
Further, the inclination and thickness of each conical plate-shaped part may be varied so that a flexible system for transferring bearing loads is achieved.
One problem with prior known solutions is that the positions of the two bearings are set from rotordynamical requirements. One of the bearings (or both bearings) may then end up in a position far from the most favourable position with regard to loads and torsional moments; vertically below the engine mount. One advantage with the inventive solution is that the design of the bearing support structure may be amended easily by replacing one of the conical plate-shaped parts with another part having a different inclination and/or length and/or material.
It is also desirable to achieve a gas turbine engine with a bearing support structure “between radially inner first and second bearings adapted to rotatably support a first and a second rotor, respectively, and a radially outer engine frame, which structure has a sufficient stiffness for transferring bearing loads and is easy to install and remove from the engine. The bearing support structure is preferably arranged between the high pressure turbine and the low pressure turbine in the direction of the engine longitudinal central axis.
Further advantageous embodiments and further advantages of the invention emerge from the detailed description below.